PROJECT 1 The Genetic and Epigenetic Basis for FSIHD We have demonstrated that FSHD is caused by a contraction-cfepencfenf (FSHD1) or contractionindependent (FSHD2) change in chromatin structure of D4Z4 only when this contraction occurs on a specific genetic background (4qA161). This leads to the hypothesis that a change in D4Z4 chromatin structure on the 4qA161 haplotype is essential for FSHD pathology. Therefore, the long-term goal is to identify the specific DNA sequences and the epigenetic modifications that together confer pathogenicity to 4qA161. Aim 1 will identify and functionally characterize the disease haplotype-specific sequence variants of the distal repeat unit and flanking pLAM sequence. Recent studies identified this part of the FSHD locus as the minimal essential region; Aim 2 will identify and functionally characterize the chromatin structure of this minimal essential region test the hypothesis that the D4Z4 repeats regulate DUX4 expression and have a biological role in early embryonic development; and Aim 3 will determine the genetic and epigenetic characteristics of D4Z4 in human ES cells to establish the developmental role of D4Z4 in relation to the clinical features of FSHD.